The present disclosure relates generally to switching-mode power supplies and control methods with regard to primary side control and load compensation.
A power supply need provide a steady output voltage at an output power node to power a load. The regulation of the output voltage is commonly achieved by using detection devices, such as resistors and LT431, at a secondary side to detect the output voltage and then passing the detection result to the power controller at a primary side with the help of a photo coupler. This kind of control means is generally referred to as secondary side control.
To eliminate the need of the detection devices at the secondary side and save the electric power there consumed, primary side control (PSC) is developed. PSC achieves the detection of the output voltage at the primary side, employing the theory of inductance coupling.
FIG. 1 demonstrates switching-mode power supply 8 using PSC. Power supply 8 includes a flyback topology 10, which uses a transformer with primary winding PRM, secondary winding SEC, and auxiliary winding AUX to isolate the primary side from the secondary side. As shown in FIG. 1, the primary and secondary sides have different grounds, isolated by the transformer. By switching power switch 15, power controller 18 controls the energizing and de-energizing of the transformer. During a discharge time TDIS when the transformer is de-energizing, secondary and auxiliary windings, SEC and AUX, discharge to charge output power node OUT and operation power node VCC, respectively. Because of inductance coupling, during discharge time TDIS, the cross voltage VSEC across secondary winding SEC should be in certain proportion to the cross voltage VAUX across auxiliary winding AUX. Power controller 18 detects cross voltage VAUX via feedback node FB, and voltage-dividing resistors 13 and 14, equivalently detecting cross voltage VSEC, which in a way is substantially equivalent to output voltage VOUT at output power node OUT. Based on feedback voltage VFB at feedback node FB, power controller 18 modifies compensation voltage VCOM at compensation node COM and accordingly controls the ON time, the OFF time, or the duty cycle of power switch 15. Simply put, PSC monitors cross voltage VAUX across auxiliary winding AUX to regulate output voltage VOUT.
PSC might induce a phenomenon that the regulated output voltage VOUT varies while load 20 is changed. It is because that parasitic resistance exists inevitably between output power node OUT and secondary winding SEC, such that output voltage VOUT is somehow smaller than cross voltage VSEC and the voltage difference there between increases along with the increase of output current IOUT. In other words, to make output voltage VOUT substantially independent from output current IOUT, the target voltages that cross voltages VSEC and VAUX are controlled to approach shall increase as load 20 or output current IOUT increases, such that the voltage difference between output power node OUT and secondary winding SEC is compensated. This kind of control concept for voltage regulation is generally referred to as load compensation.
Load compensation introduces a positive feedback loop, which, if not well designed, might cause oscillation easily. According to load compensation, for a certain load 20, the higher output current IOUT, the higher target voltages that cross voltages VSEC and VAUX are controlled to approach. Nevertheless, the higher target voltages also need further higher output current IOUT to support, such that a positive feedback loop is formed. The oscillation that would company with a positive feedback loop should be avoided or damped, however, for good output voltage regulation.